The invention relates to a method for modifying the resolution of a digitised image.
The digitising of images makes it possible to transmit and store the said images with no risk of degradation. It is mainly used in facsimile machines.
The image of a document to be transmitted is digitised pixel by pixel (or element by element) by a reading head of a scanner with which a digital value representing the intensity of grey shade read from the document is allocated to each pixel position. For restitution in black and white the shade value is compared with a mid-grey threshold to provide a 0 or 1 bit representing white or black.
In the receiving fax machine, a printer reconstructs the pixels, black or white, with the numbers of pixels per millimetre, horizontally and vertically, identical to those of the analysing head, for example, 8 pixels/mm horizontally and 7.7 pixels/mm vertically.
These numbers of pixels determine the resolution of the reconstructed digitised image and are a compromise between obtaining a good quality image at a high resolution and the transmission of a limited number of bits.
However, for some time now printers with improved resolution, such as laser printers, have been in existence. The Applicant has thus sought to use these printers to improve the resolution of the restored image.
Until then, when these laser printers were used to print a fax message its resolution was degraded in order to match it to that of a fax machine. More precisely, as the printer can only be controlled according to its own resolution, for example, 12 pixels/mm, a given number of consecutive pixels of the received image (8 pixels/mm horizontally) were transformed into a greater number of pixels representing in total the same image length (for example, from 2 to 3). This created a distortion since, continuing with the example given above, in the case of the presence of two original pixels of different values, white and black, a sort of summary smoothing was produced when restoring, between these two different pixels, the third pixel in the form of a white or black pixel, while an average, or grey, should in fact have been restored from the other two. In other words, the "grain" of the image was locally increased or decreased and this distortion degraded the image. Furthermore, for the practical reasons of volume of equipment and calculation, this above method can only be applied for simple ratios in the change of resolution.
The Applicant has thus sought to exploit fully the resolution of the printer by changing the resolution of the received image in a way which is independent of the relationship between the initial and final resolutions, horizontally and vertically, while limiting the distortion introduced by this operation.